1. Field of the Invention
The present invention relates to a semiconductor light-emitting device and a method for fabricating the same, and more particularly, to a GaN based Group III-V nitride semiconductor light-emitting device and a method for fabricating the same.
2. Description of the Related Art
Compound semiconductor based light-emitting diodes or laser diodes capable of emitting short-wavelength visible light are widely known. In particular, a light-emitting device (light-emitting diode) or laser diode fabricated using a Group III nitride semiconductor has received considerable attention because the Group III nitride semiconductor is a direct transition type material (direct bandgap material) emitting blue light at high efficiencies by the recombination of electrons and holes.
Referring to FIG. 1, a conventional light-emitting diode (LED) based on GaN based III-V nitrides includes an n-type GaN layer 12 on a sapphire substrate 10. The n-type GaN layer 12 is divided into first and second regions R1 and R2. The first region R1 has a larger width then the second region R2 and is not affected by etching after having been formed. Meanwhile, the second region R2 is thinner than the first region R1 because it is affected by etching after having been formed. As a result, there exists a step between the first and second regions R1 and R2 of the n-type GaN layer 12. An active layer 16, a p-type GaN layer 18, and a light-transmitting p-type electrode 20 are sequentially formed on the first region R1 in the n-type GaN layer 12. A pad layer 22 for use in bonding in a packaging process is formed on the light-transmitting p-type electrode 20. An n-type electrode 14 is formed in the second region R2 of the n-type GaN layer 12.
In FIG. 2, a conventional GaN based III-V nitride semiconductor laser diode in which n-type and p-type electrodes are arranged to face the same direction, and a ridge is formed in a region where the p-type electrode is formed, is shown. In the semiconductor laser diode, In particular, referring to FIG. 2, an n-type GaN layer 12, which is divided into first and second regions R1 and R2, is formed on a sapphire substrate 10. The first region R1 is wider and thicker than second region R2 so that there exists a step between the first and second regions R1 and R2. An n-type electrode 14 is formed in the second region R2 of the n-type GaN layer 12. An n-type AlGaN/GaN layer 24, an n-type GaN layer 26, and an InGaN layer 28 acting as an active layer, for which the refractive index increasingly higher in the upward direction, are sequentially formed on the first region R1 of the n-type GaN layer 12. A p-type GaN layer 30, a p-type AlGaN/GaN layer 32, and a p-type GaN layer 36, for which the refractive index is increasingly lower in the upward direction, are sequentially formed on the InGaN layer 28. The p-type AlGaN/GaN layer 32 has a ridge (or rib) at the center thereof, and the p-type GaN layer 36 is formed on the ridge of the p-type AlGaN/GaN layer 32. The entire surface of the p-type AlGaN/GaN layer 32 is covered with a passivation layer 34. Here, the passivation layer 34 extends to the p-type GaN layer 36 such that the current threshold is reduced. That is, the passivation layer 34 covers both edges of the p-type GaN layer 36. A p-type electrode 38 is formed on the passivation layer 34 in contact with a top surface of the p-type GaN layer 36, which is not covered by the passivation layer 34.
For a conventional light-emitting diode or laser diode based on a GaN based III-V nitride semiconductor in which the n-type and p-type electrodes are arranged to face the same direction, a bonding process with two wires should be performed on the same plan in a packaging process. Thus, the packaging process is complex and increases time consumption. The n-type electrode is formed in a deeply etched region so that a large step exists between the n-type and p-type electrodes, thereby increasing failure in packaging processes. As described with reference to FIGS. 1 and 2, in terms of the structure of the second region R2 of the n-type GaN layer 12, the n-type GaN layer 12 is etched to form the second region R2, for the light-emitting diode of FIG. 1, after the formation of the p-type electrode 20 or the p-type GaN layer 18, and for the laser diode of FIG. 2, after the formation of the p-type AlGaN/GaN layer 32. In other words, to form the n-type electrode 14 on the second region R2, an additional photolithography process is required, thereby increasing the manufacturing time of light-emitting devices.
FIG. 3 shows another conventional GaN based III-V nitride semiconductor laser diode in which an n-type electrode and a p-type electrode are arranged to face opposite directions with an active layer therebetween. An n-type GaN layer 12, an n-type AlGaN/GaN layer 24, an n-type GaN layer 26, an InGaN layer 28 acting as an active layer, a p-type GaN layer 30, a p-type AlGaN/GaN layer 32, and a p-type GaN layer 36, a passivation layer 34, and a p-type electrode 38 are sequentially formed on a silicon carbide (SiC) substrate 10a (or a gallium nitride (GaN) substrate). An n-type electrode 14a is formed on the bottom of the SiC substrate 10a. 
In general, the current threshold and the lasing mode stability for laser emission in semiconductor laser diodes are closely associated with temperature, and all quantal properties degrade as the temperature rises. Therefore, there is a need to dissipate heat generated in an active layer during laser emission to prevent a temperature rise in the laser diode. For the conventional GaN based III-V semiconductor laser diode, the substrate has a very low thermal conductivity (about 0.5 W/CmK for sapphire) so that the heat is dissipated mostly through the ridge. However, heat dissipation through the ridge is limited so that a temperature rise in laser diodes cannot be prevented effectively, thereby lowering the properties of devices.
For the conventional semiconductor laser diode shown in FIG. 2, it has been intended to dissipate heat generated in the active layer by applying a flip chip bonding technique, as illustrated in FIG. 4.
In particular, referring to FIG. 4, reference character A denotes the inverted conventional GaN based III-V semiconductor laser diode shown in FIG. 2. Reference numeral 40 denotes a submount, reference numerals 42a and 42b denote pad layers, reference numerals 44a and 44b denote first and second thermal conductive layers connected to an n-type electrode 14 and a p-type electrode 38, respectively, of the semiconductor laser diode A. Reference character M denotes a stack of material layers corresponding to the material layers 24 through 34 of FIGS. 2 and 3 stacked between the n-type GaN layer 12 and the p-type electrode 38.
As described above, heat dissipation efficiency can be improved by bonding a semiconductor laser diode to a separate heat dissipating assembly. However, bonding between the laser diode and the heat dissipating assembly increases the overall processing time. In addition, such a bonding process needs to follow a fine alignment between the semiconductor laser diode and the heat dissipating assembly, so that failure is more likely to occur, thereby lowering yield.
For example, assuming that the yield is 70%, about 4,000 pieces of laser diodes per wafer are obtained. A bonding time required for flip-chip bonding of all the laser diodes amounts about 20 hours (about 0.3 minutes each).